[llvm] [InstSimplify] Do not simplify a multi-use freeze in `simplifyWithOpReplaced` (PR #91215)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

See the LangRef:
> All uses of a value returned by the same ‘freeze’ instruction are guaranteed to always observe the same value, while different ‘freeze’ instructions may yield different values.

It is incorrect to replace one use of a multi-use freeze with the simplified value.
Proof: https://alive2.llvm.org/ce/z/3Dn9Cd

Fixes https://github.com/llvm/llvm-project/issues/91178

---
Full diff: https://github.com/llvm/llvm-project/pull/91215.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/InstructionSimplify.cpp (+4) 
- (modified) llvm/test/Transforms/InstCombine/select.ll (+28) 


``````````diff
diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 4061dae83c10f3..06e1a4957bc727 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4312,6 +4312,10 @@ static Value *simplifyWithOpReplaced(Value *V, Value *Op, Value *RepOp,
   if (match(I, m_Intrinsic<Intrinsic::is_constant>()))
     return nullptr;
 
+  // Don't simplify a use of a multi-use freeze.
+  if (isa<FreezeInst>(I) && !I->hasOneUse())
+    return nullptr;
+
   // Replace Op with RepOp in instruction operands.
   SmallVector<Value *, 8> NewOps;
   bool AnyReplaced = false;
diff --git a/llvm/test/Transforms/InstCombine/select.ll b/llvm/test/Transforms/InstCombine/select.ll
index 2efe2742ca4916..0a880fc986caf1 100644
--- a/llvm/test/Transforms/InstCombine/select.ll
+++ b/llvm/test/Transforms/InstCombine/select.ll
@@ -4580,3 +4580,31 @@ define i32 @sequence_select_with_same_cond_extra_use(i1 %c1, i1 %c2){
   %s3 = select i1 %c1, i32 789, i32 %s2
   ret i32 %s3
 }
+
+define i8 @test_replace_freeze_multiuse(i1 %x, i8 %y) {
+; CHECK-LABEL: @test_replace_freeze_multiuse(
+; CHECK-NEXT:    [[EXT:%.*]] = zext i1 [[X:%.*]] to i8
+; CHECK-NEXT:    [[SHL:%.*]] = shl nuw i8 [[EXT]], [[Y:%.*]]
+; CHECK-NEXT:    [[SHL_FR:%.*]] = freeze i8 [[SHL]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[X]], i8 0, i8 [[SHL_FR]]
+; CHECK-NEXT:    [[ADD:%.*]] = add i8 [[SHL_FR]], [[SEL]]
+; CHECK-NEXT:    ret i8 [[ADD]]
+;
+  %ext = zext i1 %x to i8
+  %shl = shl nuw i8 %ext, %y
+  %shl.fr = freeze i8 %shl
+  %sel = select i1 %x, i8 0, i8 %shl.fr
+  %add = add i8 %shl.fr, %sel
+  ret i8 %add
+}
+
+define i8 @test_replace_freeze_oneuse(i1 %x, i8 %y) {
+; CHECK-LABEL: @test_replace_freeze_oneuse(
+; CHECK-NEXT:    ret i8 0
+;
+  %ext = zext i1 %x to i8
+  %shl = shl nuw i8 %ext, %y
+  %shl.fr = freeze i8 %shl
+  %sel = select i1 %x, i8 0, i8 %shl.fr
+  ret i8 %sel
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/91215